Modelling jellyfish

Published: 04 September 2017

One of the development goals for the ecosystem model ERSEM (European Regional Seas Ecosystem Model) was improving the representation of mesozooplankton, the grazers that form the first link of the food web within the marine ecosystem. This group includes a multitude of diverse organisms like copepods, krill and jellyfish. Originally that group was represented by one generic organism type that covered the wide array of size and types of species that makes up mesozooplankton.

As such, in collaboration with some of the other MERP modules, we developed an approach based on size and characteristic traits to create more diversity within that group. This resulted into the creation of a framework to represent various mesozooplankton groups based on their carbon mass, to determine their metabolism (respiration, feeding) and their physical size to place them in the food chain; the bigger you are, the bigger the items you eat.

This was tested by choosing to model both copepods (a main item of fish diet) and jellyfish (same carbon mass but different physical size when compared to copepods). The approach was also designed to be extremely flexible so it is possible to have a variable number of size classes and traits for the organisms; up to 50 for each mesozooplankton type was tested. This allows the food web structure to be collapsed back to the original one group, if necessary.

This new development means that when running the model with several groups, it is possible to investigate the succession of differently sized organisms as well as dominance, by either the copepods or the jellyfish at different times of the year. This is quite a big step in term of modelling capacity as now ERSEM is the first model with a capacity for representing zooplankton diversity in this way and to date, no ecosystem model that can be coupled into an Earth System model has included jellyfish in their food web.

Model outputs are to be shared with scientists working on ecosystem services. Additionally, we have started model runs for the whole UK shelf with potential to look for areas where jellyfish are more predominant or areas at risk in climate change situations. And last but not least, the framework is full of potential for the inclusion of further groups and more traits to improve on the diversity within ecosystem models.